Secret-communication system



Dec. 15, 1 2 1.565.521

J. S. STONE El AL SECRET COMMUNICATION SYSTEM Filed Dec. 8, 1920 4 Sheets-Sheet 1 avwmtoz Dec. 15 1925. 1,565,521

J. s. STONE El AL SECRET COMMUNICATION SYSTEM Filed Dec. 8, 1920 4 Sheets-Sheet 2 Snow wow Ji/t/Z 510426 Stone 51* (Zara/k (mfg/"fine Dec. 15, 1925. 1.565.521

J. 5. STONE ET AL SECRET COMMUNICATION SYSTEM Filed Dec. 8, 1920 4 $heets-Sheet 3 Dec. 15; 1925- 1.565.521

J. 5. STONE ET AL I SECRET COMMUNICATION SYSTEM Filed Dec. 8, 1920 4 Sheets-Sheet 4 Patented Dec. 15, 1925.

E S' ATES JOHN STONE STONE, OF SAN DIEGO, CALIFORNIA, AND CLAUDE CUSTEB ROSE, OF EAST ORANGE, NEW JERSEY, ASSIGNOBS TO AIIIEEICAN TELEPHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK.

SECRET-COMMUNICATION SYSTEM.

Application filed December 8, 1920. Serial No. 429,187.

To all'u'hom it may concern:

Be it known that we, JOHN STONE STONE and CLAUDE CUSTER Ross, residing at San Diego and East Orange, in the counties of San Diego and Essex, and States of California and New Jersey, respectively, have invented certain Improvements in Secret- Communication Systems, of which the following is a specification.

This invention relates to the transmission of signaling waves, especially the transmission of complex waves, such as voice cur rent waves, and is particularly applicable to a system employing high-frequency carrier waves for conveying the voice waves.

It is one of the cardinal features of the invention that it may be utilized to transmit signals secretly and in such a manner that unauthorized persons, who may have access to the medium through or over which the waves are transmitted, will be unable to receive the message. In the arrangements of the invention this is accomplished by dividing the signal wave at the sending station into a plurality of bands or unintelligible fragments, transmitting each of these bands or fragments on a separate carrier, and recombining these bands or fragments at the receiving station in their original and intelligible relation. There may also be transmitted simultaneously with the message a plurality of disturbing or idle frequencies. These frequencies are preferably within the voice range and are transmitted on carrier currents having frequencies intermediate to or adjacent to frequencies utilized as carriers for the voice bands or fragments.

In transmitting the several hands into which the signal is divided, the carrier fre quency, which is modulated by any given band, is suppressed and one of the modulated side bands is also eliminated, so that the band of frequencies actually radiated or transmitted, in order to convey a given fragment of the signal, will be no wider than the original portion of the signal. The transmission of a single side band of this character will not alone produce an intelligible signal at a receiver, since the effect of a detector will be to produce low frequencies corresponding to the differences between the individual frequencies comprising the band, instead of the differences between each of those frequencies and the carrier frequency. A further degree of secrecy may also be obtained by, in some instances, transmitting the upper side band for certain fragments of the signal and, in other instances, transmitting the lower side band for other fragments of the signal. As the frequencies in the lower side band will be transmitted in inverse order with respect to the original low frequencies, an attempt to detect and combine the several side bands will result in failure, unless the receiving station is suitably equipped with special apparatus for receiving purposes. In accordance with the invention, special provision is also made at the receiving station for eliminating the effect of the idle or disturbing frequencies transmitted along with the side bands representing the individual portions of the signal. In order to detect from each side band the original low frequencies from which the side bands were derived, each side band is caused to beat at the receiving station with a frequency corresponding to the carrier frequency which was mod-- ulated at the sending station to produce the side band. As each individual transmitted band must beat with the proper carrier frequency, in order to produce the original message fragment transmitted, it is apparcut that only a station having information as to the carrier frequencies transmitted at the sending station will be able to detect the several bands making up the signal and to combine them in their proper relations. Inasmuch as it is possible, by modulating in successive steps, as will be more fully pointed out later, to transmit each side band as a band of frequencies quite remote from the carrier frequencyused at the transmitting station, it will be apparent that, even if a receiving operator had a variable fre quency source to beat with the received band, such an operator would be at a very serious disadvantage in adjusting the frequency of this source by reason of the fact that it would be impossible to determine from the band transmitted what frequency was used for the carrier at the transmitting station.

It is a further feature of the arrangements of the invention that they may be utilized to attain a higher degree of selectivity than any other arrangement of radio and carrier current signaling systems. It is well-known that where a signaling band is comparatively wide, as in the case of tele phone transmission, it is impossible to tune the receiving circuit sharply to the carrier frequency, for the reason that the resonance curve will have a sharp peak which would seriously distort the signal. recourse has been had in the past to the use of a broadly selective antenna, rather than a sharply tuned antenna, so that the resonance curve will be sufiiciently broad at the top to avoid undue distortion of the signal. This, of course, is very wasteful of the fre-. quency range, as it necessitates a wider separation between adjacent channels, in order to avoid interference. While it might seem, at first thought, that the method herein disclosed of transmitting a signal as a plurality of individual bands separately modulating individual carriers would be wasteful of the frequency range because of the employment of a plurality of channels for the transmission of a signal, as a matter of fact, the method actually permits of a very economical use of the frequency range. This follows from the fact that, by narrowing the frequency band of the signal wave transmitted by each carrier frequency to a band much less in width than the voice range, the receiving apparatus may be very much more. sharply selective for each band. Consequently, in accordance with the method of the present invention, the band of signaling frequencies, such as the voice band, may be subdivided into as many individual bands as desired, each sub division producing a narrower individual signaling band. These bands are stepped up in frequency without any increase in the width of the band and are transmitted at high frequency without the accompaniment of a transmitted carrier. The bands may, therefore, be as closely spaced during transmission as is consistent with the selective possibilities of the receiving station. At the receiving station, each selective circuit "wouldyas the subdivision increased, be capable of being more sharply tuned or, in other words, would become more highly selective. present invention, the degree of selectivity at the receiving station might be determined by the number of subdivisions of the voice frequency band at the sending station.

Other features and purposes of the invention will appear more fully from the following description when read in connection with the accompanying drawing, in which the arrangements of the invention are illustrated.

Accordingly,

Therefore,'-in accordance with the In Figures 1, 2 and 3 are illustrated sending arrangements applicable to a radio telephone system embodying the invention, while in Figs. 4 and 5 are shown receiving arrangements for such a system. Figs. 6, 7 and 8 illustrate characteristic curves portraying the principles of operation of the selective transmitting and receiving circuits. Figs. 9 and 10 illustrate circuit arrangements embodying somewhat more elaborate modifications of the transmitting apparatus, while Figs. 11 and 12 disclose receiving arrangements to be used in connection therewith.

Referring to the drawing, Fig. 1 illus trates a transmitter of. the type contemplated but reduced to its simplest form, for the purpose of making the description of its operation as free from ambiguity as pos sible. A transmitting circuit 1 is shown, comprising a telephone transmitter 2 'in circuit with a battery 3, or other source of current, and the primary of a transformer 4;, the secondary of which is included in a circuit having three branches 5, 6 and 7, said branches including band filters 13,, B, and 13,, respectively. The branches 5, 6 and 7 also include the individual field windings of high frequency generators G G and G respectively, the armatures of which are included in the transmitting antenna arrange ments T T and T This arrangement of the generator circuits is such that the current in the field windings of each generator is determined by the portion of the telephone band which passes through the corresponding filter and, consequently, the high frequency generated by'the generatorwill be modulated in accordance with the transmitted band. It will be understood, of course, that this arrangement of the gencraters (1 (i and G fis s mbolic of any known means whereby sources of current of high frequencies, for example, F F and F respeii-tively, may be modulated by currents passed through the filters 13,, B, and 13,. The radiating antenna arrangements T,, T, and 'i.,, may be made selective of the carrier "freipicncics generated by the devices G G and 9-,, by means well-known in the art, but these antennae are p iferably so ramstructed "that instead of being resonant at the parof resonant circuits 52 each com 115111! a capacity in parallel with an inductance. In each case, the inductance is made relatively small and the capacity relatively large, so that the selectances of each circuit will be small as compared with that of the aerial alone. By selectance is meant the function which may be expressed thus where l is the inductance, U the capacity and R the resistance. As fully pointed out in the co-pending application, of John Stone Stone, filed November 30, 1920, Serial No. 7,392, entitled Circuits for passing or stopping a frequency band of alternating currents, a proper proportioning of these elements will give a characteristic similar to that indicated by the curve As will be seen from this curve, between frequencies FX and F-l-X, the reactance introduced by the resonant elements will be substantially equal and opposite to that of the antennae, so that the resulting rcactance of the system as a whole will be as indicated by the curve 5-2. The reactance of the curve is substantially zero over a range offrequeneies between F-X and'F-|-X, and this range maybe made equal'to that of the band produced by modulation. For frequencies beyond this band, the reactance increases rapidly. 13y constructing each of the antennae T,,T and T, in this manner, the/antennae may be caused to produce a minimum degree of distortion for the barul of fre quencies each antenna is to transmit.

It will be seen that by the arrangement thus far described. the frequency band transmitted on each carrier is relatively narrow, so that it is possible, in the case ofthe relatively low frequency bands. to use at the receiving station. circuits selective of the transmitted carrier frequencies, which are very sharply tuned thereto. as compared with the tuning of a circuit adapted to receive a carrier modulated by an entire voice transmission band. The increased sharpness of tuning permits of a closer spacing of the carrier frequencies and; likewise, of a closer spacing of adjacent carriers used for other channels. The arrangement also renders the transmitted message unintelligible to an ordinary receivin station unprovided with special equipment for receiving from a sending station of this character, since, if the antenna of the ordinary receiving station be tuned to one of the-carrier frequencies F F or F it will only receive the portion of the voice band transmitted by that carrier and the'other portions of the message will not be present.

A further refinement of the secrecy-- feat'ure' may be obtained by employing simultaneously at the sending station, in addition to the arrangments illustrated in Fig. 1, the arrangements illustrated in Fig. 2. In this figure, a transmitting circuit 8 is shown, comprising a telephonetransmitter 9 in circuit with a battery 10 and the primary of a transformer 12, the secondary of which is in circuit with branches 13 and 1 1, including the individual field windings of generators G and G the armatures of which are included in transmitting antennaarrangements T,, and T As in the case of Fig. 1, the arrangements Gr and Gr; are symbolic of any known devices whereby high frequency currents having frequencies, for example, F and F' may be modulated by currents from the telephone transmitting apparatus 8. The radiating arrangements T and T may be tuned respectively to the frequencies F and F which are preferaby medial fre quencies to the frequencies F F and F F respectively. The term medial, as here used, has referenee to the approximate geometrical means as, for example, /F F and /IF F It'will be understood, however, that the invention is not limited to this arrangement of the frequencies. A phonographic apparatus 11 is arranged so that the sound of noise from a phonographic record may be spoken into the transmitter and radiated by the radiating devices T and T along with the message radiated by the radiators T T and T The sound produced by the phonographic apparatus should be of a purely arbitrary character and, in this connection, it may be found useful to run the phonograph backwards, in order to make the noise more unintelligible.

7 As previously pointed out, the arrangement shown in Fig. 1, when used alone, has certain advantages from the standpoint of secrecy, and, consequently, requires specially arranged receiving apparatus in order to obtain an intelligible message. The messagetransmitted by the transmitter of Fig. 1' might, for example, be received by means of the arrangement illustrated in Fig. 5. In this figure, R R and designate receiving aerials which may be rendered selective of frequencies F F and F, by means well known in the art, but which are preferably constructed in a manner similar to the transmitting aerials T T and T, already described, so as to have a reactance characteristic such as that illustrated by the curves of Fig. 8. Each of these aerials will'then be selective of a band of frequencies of the desired width and of the character radiated from the corresponding transmitting antennee. For example, the aerial B, may be selective of the band of frequencies transmitted by the antenna T etc. The receiving antenna will tend to discriminate against frequencies lying without its parthe transformer, so that the entire band wlll be transmitted to the telephone receiver.

The three aerials-T T and '1, may be so adjusted as to their radiation and the three generators may be so adjusted as to their power and modulation, that the three bands of frequencies passed respectively by the filters 13,, B and B are transmitted proportionately.

Thus, if p p and p be the radiation resistance, respectively, of the three radiators at the frequencies F F and F respectively, and if 1,, 1,, 1 be the amplitudes of the modulation of these frequencies, respec tively, then we may have lfp ilgp iIgp W1 W ZW where VV and are the energies of the telephone currents passed by the filters B B and B respectively. These adjustments, however, are not to be made by calculation and must be made through symmetry of the apparatus and by extremely simple adjustments which present little difliculty.

Similarly, at the receiving station, through symmetry of the apparatus and simple adjustments, the three frequencies may be received in such proportionalityto that. in which they are transmitted, that the audio frequencies developed in the'audio frequency circuit of each detector will correspond, in quantity as well as quality, to the bands passed by the filters B B and B respectively, at the transmitter. 7

It may, under some circumstances, however, be desirable to depart from such a symmetrical. adjustment, and to transmit the frequencies F F and F at very different intensities, so as to render the reception of the message still more difficult at a receiver to which the message was not addressed.

Thus, it may be desired to transmit the bands passed by filters B B and 13,, respectively, on carrier frequencies F F and F at amplitudes m m and m respectively. In this case, the receiver must be so adjusted that the coupling coefficients and of the detectors 1",, r, and r, to the aerial, are in the proportion or the coupling of the audio frequency circuits of these detectors to the telephone T, may be in such proportion.

It is readily apparent from the preceding discussion, that there is a remote possibility that a message of the character transmitted by the station of Fig. 1 might be received by a station for which it was not intended. If such a station were provided with a plurality of receiving antennae, a receiving operator might, by the exercise of considerable patience, tune the several antennae to the different carriers transmitted by the stationof Fig. 1 and by painstaking adjustments, combine the individually detected portions of the telephone band in such proportions asto magnitude or intensity of the corresponding currents as to reproduce the telephonicsig'nal transmitted from the distant station. VJhcre, however, the signal is further confused by the employment of the transmitting apparatus in Fig. 2, the reproduction at a receiving station of the actual signal intended to be transmitted may be rendered practically impossible, even by the employment of receiving apparatus illustrated in Fig. 5, since the carrier frequencies F, and

F upon which the disturbing noise is,

transmitted, may be very closely spaced with respect to the frequencies F F and F upon which the message is transmitted. Consequently, the receiving antennae R R and 1%,, even though they are sharply tuned,

will receive very considerable interference from the intermediate frequencies F and and the detected message will be mingled with unintelligible noise.

In order to receive an intelligible message when transmitted by the arrangements of Figs. 1 and 2, under the aforesaid conditions, the apparatus illustrated in Fig. 4: may be employed. As it is not essential or indeed desirable that there be separate aerials for each of the frequencies F F and F, at the receiver, the receiving device in Fig. l is 4 if, in addition, they are resonant respectively at frequencies F F and F and if the aerial per so be made resonant at frequency F,, then the three resonant branches 8,, S and S, will give a reactance frequency function curve corresponding to the curve 1 of Fig. 6. If, new, the aerial per se as tuned to the frequency has a lesser selectivity and its reactance be represented by the curve 2 o ig- 6, the resu ng reaetan nf the system will be represented by the dotted line curve 8, which is obtained by adding the reactances indicated by the curves 1 and An examination of this curve shows that points of zero reactance correspond to frequencies F,, F, and F while points of very great reactance correspond to the intermediate f requencies F and F Consequently, if the antenna arrangement is so designed that the frequencies F F and F correspond to the three carrier frequencies used at the transmitting station of Fig. 1 and that the frequencies F and F correspond. to the carrier frequencies employed by the transmitting arrangement of Fig. 2, it will be apparent that the receiving antennae will have such a characteristic as to be sharply selective of frequencies in the neighborhood of F F and F, but will ofier very considerable impedance of frequencies in the neighborhood of F and F As a consequence, the receiving antennae system will discriminate againstthe carrier frequencies carrying the disturbing noise, while it 'Will readily receive the three fre quencies carrying the divided message. Such a receiver Will, therefore, enable the operator to receive the intelligible message to the exclusion of the unintelligible noise superposed upon it and stations not so arranged as to receive from the transmitting apparatus of Figs. 1 and 2, will be unable to receive an intelligible message. In order to detect the low frequency signaling bands from the radiated bands received by the antenna arrangement R, the antennae may be coupled through a transformer 40 and a circuit 41 having branches including band filters 13,, B and 13,. These band filters will select the bands radiated by the antennae T T and T and will discriminate against the bands radiated by the antennae T and The selected bands will be impressed upon. detectors 1),, D and D of any well known type, which may be included in the branch circuits. The detected low frequency currents may be impressed upon a common circuit 4% through a transformer arrangement 43 and thus combined in intelligible relation at the receiver 15.

lVhile, as illustrated in F igs. 1 and 2, a plurality of radio antennae are employed for transmitting the several fragments of the signal, it is not necessary and, in general, is not desirable to employ a plurality of antennae, but a transmitting antenna may be employed, which is provided with a single aerial having a plurality of degrees of freedom corresponding to the frequency bands to be transmitted. An antenna of this character is illustrated in Fig. 3, in which T is a transmitting aerial having branches 8,, S and S each tuned to a different frequency by means of a suitable arrangement of inductance and capacity. If the individual branches are arranged to have equal seleocances and if they are resonant respectively at frequencies F F F F and F respectively, then, if the aerial per se be made resonant at frequency F the several resonant branches S to S inclusive will give a reactance frequency function curve corresponding to the curve 1 ofFig. 7. If, now,

the aerial per so as tuned to the frequency F," has a lesser selectivity and its reactance be represented by the curve 2 of Fig. 7, the resulting reactance of the system Will be represented by the dotted line curve 3. An examination of this curve shows that points of zero reactance correspond to frequencies F F F F and F If these frequencies be so chosen as to correspond to the medial frequencies of the bands to be transmitted, as hereinafter described, it will be apparent that the transmitting aerial T will have large radiating capacity at frequencies in the neighborhood of the points of Zero reactance but will discriminate against frequencies remote therefrom. The antenna T may be coupled through a multiple transformer arrangement 00 toa plurality of circuits including the generating and modulating arrangements G G G G and G these modulating arrangements being similar to those illustrated in Figs. 1 and 2, and having modulating bands impressed upon them in the same way, as illustrated in said figures.

Fig. 9 illustrates arrangements for elimihailing from the transmitted frequencies not only the carrier frequencies involved, but also one side band of each pair of side bands, thereby increasing the selective possibilities and also providing for additional secrecy. Referring to Fig. 9, T is a transmitting aerial similar to that already described in connection with Fig. 3 and having similar characteristics. The modulating arrangements employed in Fig. 9 are somewhat different from those illustrated in Figs. 1 and 2. In this instance, the circuit of the transmitter 1 is associated with branches 5, (3 and 7 leading through band filters 13,, B and B to modulating devices M M and M,, respectively. These modulating devices are preferably of the balanced vacuum tube type disclosed in the U. S. patent to John E. Carson, N 0. 1,343,307, dated June 15, 1920. This type of modulator is arranged to suppress the carrier frequency involved and transmit only the side bands. It is desirable, in order to provide for greater economy in the use of the frequency range, to transmit only one side band and, consequently, the carrier frequencies F F and F 3 supplied to the common branch of the input circuit of. each modulator are so chosen as to be relatively low, so that it will be possible to effectively discriminate between the two side bands by means of suitable filtering apparatus. For the purpose of thus discriminating, band filters BF BF and BF are associated with the output circuits of the modulators. The filter BF, may be arrangedto transmit a band of frequencies F -l-B where B represents a narrow band of frequencies corresponding to a portion of the signal and F is the carrier frequency involved. It will be seen that this represents a band whose width is the same as that of B and that the various frequencies in this band are all above the carrier frequency F Similarly, the band filter BF may be arranged to transmit the side band F +B While it is possible to arrange the filter BF to also transmit the upper side band produced in the output circuit of the modulator M it might be, in some instances, desirable to transmit the lower side band F -B as this inverts the order of the frequencies involved in the transmitted band and produces further confusion in the signal, which can only be eliminated at the receiving station by special arrangements. The bands transmitted from the filters BF and BF may be impressed upon modulators M M and M similar in construction and arrangement to the modulators M M and M already described. These modulators are supplied with carrier frequencies F F and F which may be of the order of radio frequencies and these frequencies are modulated by the bands transmitted from the band filters to produce side bands, each having a width corresponding to the original bands transmitted through the filters B B and B respectively but spaced a considerable distance in frequency from the carriers F F and F since the frequencies in each band are commensurate with the lower carrier frequencies F F and F employed in the first step of modulation. Band filters BF BF, and BF are provided in the output circuits of these modulators for the purpose of transmitting one side band and suppressing the other. By designing these filters so that the transmitted band has a width substantially no greater than the one side band to be trans mitted, the filters will also serve to suppress the carrier frequencies although these frequencies are, in the case illustrated, suppressed by the balanced arrangement of the modulator circuit. The band filters are arranged, in the case illustrated to transmit the upper side band although, in practice, the lower side band may be transmitted, if desired. Accordingly, the filter BF, is arranged to transmit a band F,+F +B,, these having a width equal to B and representing a band of frequencies in the neighbor hood of F +F Similarly,the filter BF will transmit a band F +F +B while the filter BF, will transmit a band F +F -B The three bands transmitted may be impressed upon a common circuit 15 for transmission to the radiating antenna and it will be readily apparent that the carrier frequencies may be so chosen as to bring the several bands as closely together in the frequency range, as is desired.

In order to transmit unintelligible noise along with the three signal bands previously described, the phonographic transmitting apparatus 8-11 is connected to the circuit having branches 13 and 14:, including band filters B and B These band filters transmit the disturbing noise currents to modulators M and M similar to the modulators already described, and these modulators are supplied with carrier frequencies F and F which frequencies may preferably be inter mediate or adjacent to the frequencies F F and F F Filters BF and BF are associated with the output circuits of the modulators M and M for transmitting side bands F .,+B and F +B respectively. These side bands are impressed upon modulators l\ and M supplied with carrier frequencies F and F Filters BF {and BF associated with the output circuits of these modulators, may be arranged to select one of the resultant side bands and discriminate against the other side band and the carrier frequency involved in each case. For purposes of illustration, the filter BF! is indicated as transmitting a band F +F B and the filter BF is illustrated as transmitting a band F +F +B These bands, like the bands transmitted by the filters BF BF and BF will be of relatively narrower width and preferably will be somewhat remote from the carrier frequencies supplied to the modulators and the carrier frequencies will not be transmitted along with the bands. All five bands may be impressed upon the common circuit 15, which is associated through a transformer 16 with the transmitting antenna T. Referring again to the curve 3 of Fig. 6-, which represents the transmission characteristic of the transmitting antenna, the frequencies F F F and F corresponding to points of zero reactance, should be so'chosen as to be medial. frequencies for the bands transmitted by the filters BF BF], BF BF and BF respectively.

An advantage which is inherent in the arrangement of Fig. 9 resides in the possibilities which it affords of so choosing the frequencies supplied to the various modulators that the bands ultimately radiated from the radiating apparatus T may be arbitrarily and widely spaced in the frequency spectrum. It is also possible to shift the relative positions of the bands in the frequency spectrum. For example, the band passed by the filter B may be intermediate in the voice range, between the band passed by the filter B and that passed by the filter B By a proper choice of the carrier frequencies, however, the high frequency bands corresponding to these bands may be transposed so that the band representing the signal passed through the filter B, will be higher than that representing the signals passed through the filters B and B Other bands from other transmitting statitns may be so arranged as to be intermc Mite between the several bands radiated by the transmitting apparatus of Fi 9 and in this Way, the entire available frequency range may be utilized and a further degree of secrecy attained, owing to the fact that a receiver, tuned to receive one of the bands radiated by the transmitter of Fig. 9, for example, would also receive adjacent bands on either side of it which originated as parts of signals transmitted from other transmitting stations. Such an arrangement of the bands of the frequency spectrum would permit, therefore, of using bands from other radiating stations as the idle frequencies for rendering the signals transmitted from the station of Fig. 9 unintelligible so that it would not be necessary to provide the idle frequencies from the transmitting apparatus 8, although this apparatus may be provided as an additional precautioin desired.

A receiving apparatus adapted to receive intelligible signals from the transmitting arrangement disclosed in Fig. 9 is illustrated in Fig. 11. This receiving arrangement comprises a receiving antenna R, having a transmission characteristic similar to that indicated in Fig. 4, said antenna being associated. through a transformer 17 with a circuit 18 having branches leading through filters BR', B'F and BF By proper design of the antenna, the points of zero reactance may be made to correspond to medial frequencies of the bands transmitted by the filters Bi BF; and BF at the transmitting station, while discriminating against the bands transmitted by the filters BF; and BF The bands to which the antenna arrangement is approximately selective are individually selected by he band filters BF BF and BF respectively, at the receiving station and impressed upon demodulators D D and D These demodulating devices may be of the balanced vacuum tube type described in the U. S. patent to John R. Carson, No. 1,23%,308 issued June 15, 1920 and are supplied with locally generated carrier fre quencies F F and F respectively, corresponding to the carrier frequencies used in the last step of modulation at the transmitting station. is a result of the detecting action of the demodulators, frequencies iiequency will be applied. to similar demodulating devices D D. and D respectivelj; These demodulating devices are upplied with locally generated carrier frequencies l F and F correspouu to the frequencies used in the first step of modula tion at the transmitting station. The dc modulator D functions to subtract the band l ll from the carrier frequency F and the demodulators D and D function to subtract the carrier frequencies from the corresponding hands zuvplied to the lemodu lators. bands resulting from the subinc traction of the carri frequency from the band. applied to the demodulator will, in each case, correspond to the corresponding portions of the signal band at the transmitting station and this low frequency band may, in each case, be selected from the upper band by means of filters L8,, B and B], which are connected to a common rece 'ing circuit 19, so that the component bands of the signal may be combined in the receiver 20.

A. somewhat similar arrangement can bodying the same principles is illustrated in ll) and 12, the former illustrating the transmitting apparatus and the latter the receiving apparatus. Referring to Fig. 10., it will be seen that the appara is involved in the first step of modulation may be identical with that of Fig. 9. but in this instance the output circuits BF, to BF inclusive, are connected to a common circuit 21. which is associated with the input circuit of a modulator M of the general type already described. This modulator may be supplied with a carrier frequency F of radio character. is a result f the action of the modulator M, the five relatively narrow bands appearing in the circuit 21 are added to and subtracted from the frequency l thus producing two sets of bands on either side of carrier frequency F, and considerably spaced therefrom. A filter BF is provided in the output circuit of the modulator Pal and this filter designed to suppress the bands on one side of ti 2 carrier and transmit those on the other. in the case assumed, the filter is desi ned to transmit the upper set of bands and suppress the carrier frequency and the lower set of bands. The upper set of bands in this case comprises band frequencies F +B and F'+F +B The transmitting antenna 1 in this case may be constructed in accordance with the principles illustrated in Fig. 8 to have zero reactance over a band of frequencies of suf'ficient width to accommodate the several bands impressed upon it through the band filter BF, and, therefore,

will readily transmit the particular frequencies desired While tending to discriminate against other frequencies.

The frequencies transmitted may be all closely spaced, owing to the highly selective character of the receiving apparatus cnuiloyed at the receiving station, which is illustrated in Fig. l2 and will now be deecrihcd. In this case, the receiving antenna B may 'i.ven the characteristics indicated in rig. 8 so as to have zero reactance over a hand of frequencies corresponding to the hand radiated by the antenna T of Fig. 1. It will, therefore, readily receive the frequencies included in the five bands of frequencies transmitted in the as sumed'case but will discriminate against other equencies. The receiving antenna it is associated with the input circuit of a demodulator D, which is supplied with the carrier frequency ll", corresponding to the carrier frequency used in the last step of modulation at the transmitting station of Fig. 10. As the result of the action of the demodulator D, the frequency F is added to and subtracted from the frequencies in each of the bands radiated by'the transmits ting antenna T. A band filter BF is associated with the output circuit of the demodulator, however, and this filter is so designed as to eliminate the upper set of frequencies and pass the lower set of frequencies, which will comprise the bands I A-B and F d-B as well as the intermediate bands. It will be seen that these bands include the disturbing bands, and in order to eliminate these disturbing bands and to select the desired bands, the frequencies represented by all five bands are impressed upon the common circuit 28 having branches leading through filters Bll Bli, and BF These filters function to suppress the two undesired bands and to select and separate the three desired hands into separate circuits and impress them upon the demodulators D D and D which function in a manner identical with the corresponding elements in Fig. 9 to detect the component bands of the signal. and combine them in the receiver 20.

It will be observed that in this case the received bands are all stepped down in frequency before any selecting action takes place, so that the degree of separation between the bands becomes relatively large, as compared with the actual frequencies then involved, so that it is much easier to discriminate between the bands desired and much. more closely together than has heretofore been the practice.

l'l' hile the invei'ition has been disclosed as pr: cularly' applicable to a radio s stem it is understood t vention are equally well applicable to a system utilizing wire lines. It will be obvious that the general principles herein disclosed may be emhodie in many other organizations widely dini'erent from those ilhistratcd, without departing spirit of the invention, as defined in the following claims.

vi la-at is claimed is:

l. l n a radio transmission system, a sendtation, means at said sending station for iding the mess 'e frequency band into sub-bands and for radiating each of said sub-bands on separate carrier frequency, means at said sending station for producing L rbing low frequency band, and means for radiating said low frequency band on carrier frequencies intermediate in the frequency to said first mentioned carrier frequencies.

2. in a radio telephone system, a sending station, means thereat for dividing the voice frequency band into unintelligible fragments and for transmitting each of said fragments on separate carrier frequencies, means for producing a low frequency band within mt the principles of the infrom the 7 the voice range of frequencies, and means Y message frequency band into sub-bands and for radiating each of said sub-bands on a separate carrier frequency, means for producing an additional low frequency band means for radiating said additional low frequency band on a carrier frequency intermediate in the frequency scale to said first mentioned carrier frequencies, and a receiving station including means adapted to receive only the carrier frequencies transmitting the sub-bands of the message frequency band, and means for combining said sub-bands in their original relation.

at. In a radio transmitting system a sending station including means for dividing the message frequency band into sub-bands and for radiating each of said sub-bands on a separate carrier frequency, means for pro ducing an additional low frequency band, means for radiating said additional low frequency band on a carrier frequency intermediate in the frequency scale to said first mentioned carrier frequencies and a receiving station including an arrangement of circuits so tuned that only said first mentioned carrier frequencies will be detected.

5. The method of radio transmission which consists in dividing the message frequency band at the sending station into subbands, radiating each of said sub-bands on a separate carrier frequency, producing at the sending station a disturbing low frequency band, and radiating said low frequency band on carrier frequencies adjacent in the frequency scale to said first mentioned carrier frequencies.

6. In a radio transmission system, a sending station, means at said sending station for dividing the message frequency band in to sub-bands, means for translating each sub-band into a band of radio frequencies having a width no greater than that of the corresponding sub-band, whereby each band when transmitted may be received to the exclusion of other bands by ordinary sharply tuned selective apparatus, without appreciable distortion due to the width of the band.

7 In a radio transmission system, a sending station, means at said sending station for dividing the message frequency band into sub-bands, means for translating each sub-band into a band of radio frequencies having a width no greater than that of the corresponding sub-band, whereby each band when transmitted may be received to the exclusion of other bands by ordinary sharply tuned selective apparatus without appreciable distortion due to the width of the band, and means at a receiving station for selecting the individual bands thus radiated to the exclusion of other bands, and means for detecting therefrom the original sub-bands and for combining the sub-bands together to form a signal.

8. In a radio transmission system, a sending station, means at said sending station for dividing the message frequency band into sub-bands, means for translating each sub-band into a band of radio frequencies having a width no greater than that of the corresponding sub-band, and means for transmitting certain of the translated bands. with their frequency relations inverted with respect to the original sub-band, whereby each band when transmitted may be received to the exclusion of other bands by ordinary sharply tuned selective apparatus, without appreciable distortion due to the width of the band.

9. In a radio transmission system, a sending station, means at said sending station for dividing the message frequency band into sub-bands, means for modulating separate carrier frequencies in accordance with the sub-bands, and means for radiating one only of the side bands involved in each separate modulation to the exclusion of the other side band and the carrier frequency, whereby each band may be detected without distortion by a receiving station sharply tuned in the neighborhood of one of the bands and the detected signal will be unintelligible.

, 10. In a radio transmission system, a sending station, means at said sending station for dividing the message frequency band into sub-bands, means for translating each sub-band into a band of radio frequencies having a width no greater than that of the corresponding sub-bands, means for simultaneously radiating the several translated bands, and means for simultaneously transmitting other bands lying in frequency ranges intermediate to the radiated bands corresponding to the signal.

11. In a radio transmission system, a plurality of sending stations, means at each sending station for dividing a message to be transmitted from said station into sub-bands, means at each station for translating the various subbands into radio bands of corresponding width, said means being arranged so that the several bands radiated from each station will be separated from each other, and means to transmit disturbing bands which will lie intermediate between said bands of a given station, thereby producing disturbing frequencies at receiving stations which are not arranged to discriminate against undesired bands.

12. The method of obtaining secrecy in the transmission of intelligence, which consists in dividing each of a plurality of message frequency bands into sub-bands, translating each sub-band into corresponding radio frequency bands, the radio frequency bands being so arranged that sub-bands of different messages will be intermingled in the frequency spectrum.

13. The method of secret multiplex transmission, which consists in producing a number of message frequency bands, dividing each message frequency band into a plurality of sub-bands, translating each sub-band into a corresponding radio frequency band, the radio frequency sub-bands corresponding to each signal being so related to each other and to the sub-bands of other signals that sub-bands of different signals will lie adjacent to each other in the frequency spectrum.

In testimony whereof, we have signed our names to this specification this 18th day of November, 1920.

JOHN STONE STONE. CLAUDE CUSTER ROSE. 

